In Java, C and C++ I see people using intensively debugging strategies (because mostly they don't know about TDD). On the other hand, debugging too can help to understand software abstractions. So, when and how do you use debugging in Rails applications?
Its the same as everything else, you need a debugger when you don't really know where the problem is. This may be more rare if you are using TDD, but if your application is fairly complex you will never have 100% test coverage. I do find with Rails that I don't really need an integrated debugger, the console is enough.